As is known, a radial tire comprises a tread, two inextensible beads, two sidewalls connecting the beads to the tread, and a belt placed circumferentially between the carcass reinforcement and the tread. This carcass reinforcement is made up in a known manner of at least one rubber ply (or “layer”) which is reinforced by reinforcing elements (“reinforcing threads”) such as cabled threads or monofilaments, generally of the metal type in the case of tires for industrial vehicles.
To reinforce the above carcass reinforcements, it is general practice to use what are called “layered” steel cords formed from a central core and one or more layers of concentric wires placed around this core. The layered cords most often used are essentially cords of M+N or M+N+P construction, formed from a core of M wires surrounded by at least one layer of N wires, said layer itself being optionally surrounded by an outer layer of P wires, the M, N and even, P wires generally having the same diameter for simplification and cost reasons.
To fulfil their tire carcass reinforcement function, the multilayer cords must firstly have good flexibility and high endurance in bending, which means especially that their wires have to have a relatively small diameter, preferably less than 0.30 mm, more preferably less than 0.20 mm, this being generally smaller than that of the wires used in conventional cords for the crown reinforcements of tires.
These multilayer cords are also subjected to high stresses when the tires are miming, especially subjected to repeated bending or variations in curvature, which cause rubbing on the wires, especially due to contacts between adjacent layers, and therefore causing wear and fatigue. The cords must therefore have a high resistance to what is called “fretting fatigue”.
Finally, it is important for them to be impregnated as far as possible with the rubber that this material can penetrate into all the spaces between the wires constituting the cords. Indeed, if this penetration is insufficient, empty channels are then formed along the cords, and corrosive agents, for example water, liable to penetrate into the tires, for example as a result of cuts, travel along these channels right into the tire carcass. The presence of this moisture plays an important role, causing corrosion and accelerating the above degradation process (“corrosion fatigue” phenomena) compared with use in a dry atmosphere.
All these fatigue phenomena can generally be grouped under the generic term “fretting corrosion fatigue” and cause progressive degeneration in the mechanical properties of the cords and may affect the lifetime of said cords under the severest running conditions.
On the other hand, the availability of carbon steels of ever greater strength and endurance means that tire manufacturers nowadays are tending, as far as possible, to use cords having only two layers, in particular so as to simplify the manufacture of these cords, to reduce the thickness of the composite reinforcing plies, and thus reduce tire hysteresis, and ultimately to reduce the cost of the tires themselves and the energy consumption of vehicles fitted with such tires.
For all the above reasons, the two-layer cords most often used at the present time in tire reinforcement carcasses are essentially cords of 3+N construction formed from a core or inner layer of 3 wires and an outer layer of N wires (for example, 8 or 9 wires), the assembly optionally being able to be hooped by an outer hoop wire wound in a helix around the outer layer.
As is known, this type of construction promotes the penetration of the cord from the outside by the calendering rubber of the tire or other rubber article during the curing thereof, and consequently makes it possible to improve the fretting/corrosion-fatigue endurance of the cords.
Moreover, it is known that good penetration of the cord by rubber makes it possible, thanks to a lesser volume of trapped air in the cord, to reduce the tire curing time (“reduced press time”).
However, cords of 3+N construction have the drawback that they cannot be penetrated right to the core because of the presence of a channel or capillary at the centre of the three core wires, which channel or capillary remains empty after external impregnation by rubber and is therefore propitious, through a kind of “wicking effect”, to the propagation of corrosive media such as water. This drawback of cords with a 3+N construction is well known, being discussed for example in the patent applications WO 01/00922, WO 01/49926, WO 2005/071157 and WO 2006/013077.
To solve this core penetrability problem of 3+N cords, patent application US 2002/160213 proposes to produce cords of the in-situ-rubberized type.
The process described in this application consists in individually sheathing (i.e. sheathing in isolation, “wire to wire”) with uncured rubber, upstream of the assembling point of the three wires (or twisting point), just one or preferably each of the three wires in order to obtain a rubber-sheathed inner layer, before the N wires of the outer layer are subsequently put into place by cabling around the thus sheathed inner layer.
This process poses many problems. Firstly, sheathing just one wire in three (as illustrated for example in FIGS. 11 and 12 of that document) does not ensure that the final cord is filled sufficiently with the rubber compound, and therefore fails to obtain optimal corrosion resistance and endurance. Secondly, although wire-to-wire sheathing of each of the three wires (as illustrated for example in FIGS. 2 and 5 of that document) it does actually fill the cord, it results in the use of an excessively large amount of rubber compound. The oozing of rubber compound from the periphery of the final cord then becomes unacceptable under industrial cabling and rubber coating conditions.
Because of the very high tack of uncured rubber, the cord thus rubberized becomes unusable because of it sticking undesirably to the manufacturing tools or between the turns of the cord when the latter is being wound up onto a receiving spool, without mentioning the final impossibility of correctly calendering the cord. It will be recalled here that calendering consists in converting the cord, by incorporation between two uncured rubber layers, into a rubber-coated metal fabric serving as semifinished product for any subsequent manufacture, for example for building a tire.
Another problem posed by individually sheathing each of the three wires is the large amount of space required by having to use three extrusion heads. Because of such a space requirement, the manufacture of cords comprising cylindrical layers (i.e. those with pitches p1 and p2 that differ from one layer to another, or having pitches p1 and p2 that are the same but with twisting directions that differ from one layer to another) must necessarily be carried out in two discontinuous operations: (i) in a first step, individual sheathing of the wires followed by cabling and winding of the inner layer; and (ii) in a second step, cabling of the outer layer around the inner layer. Again because of the high tack of uncured rubber, the winding and intermediate storage of the inner layer require the use of inserts and wide winding pitches when winding onto an intermediate spool, in order to avoid undesirable bonding between the wound layers or between the turns of a given layer.
All the above constraints are punitive from the industrial standpoint and go counter to achieving high manufacturing rates.